Antiground noise recording system



Juky 11, 1939. G. L, DIMMICK ANTIGROUND NOISE RECORDING SYSTEM Filed Feb. 15, 1957 2 Sheets-Sheet 1 lnnentor (Ittomeg July 11 1939. G. LI DlMMlCK ANTIGROUND NOISE RECORDING SYSTEM 2 Sheets-Sheet 2 PHONE Filed Feb. 15, 1957 Patented July 11, 1939 UNITED STATES PATENT OFFICE Glenn L. Dimmick, Erlton, N. J assignor to Radio Corporation of America, a corporation of Delaware Application February 13, 1937, Serial No. 125,544

6 Claims.

This invention relates to a method of and apparatus for recording sound on film with a minimum of ground noise and more particularly to the provision of means for reducing the ground noise produced by the relatively opaque portion of the sound track.

In ground noise reduction systems of the prior art, such as those described in my Patent 1,999,721 and in McDowell Patent 1,855,197, the

ground noise is reduced in variable width film recording by reducing the clear area on the print which is not actually occupied by the sound track, or used for modulation. While it is true that a large part of the ground noise is caused by opaque particles or scratches in the clear area and the aforesaid patents disclose ways of reducing this ground noise, another and very objectionable source of ground noise is that due.

to the transmission of light through the dark part of the sound track. This part of the sound track is ordinarily not altogether opaque but transmits about 5 percent of the incident light. Increasing the density of the print above a value of a density of from 1.2 to 1.4 results in undue spreading of the image and a serious loss of high frequencies in the reproduced sound. Opaque particles or scratches on the negative result in transparent holes or lines in the dark part of the print. This gives rise to ground noise which is not reduced by any of the present systems which function only to reduce the ground noise originating in the clear portion of the track. This diiliculty is also encountered in connection with push-pull recording such as that described in my application Serial No. 610,302, filed May 9, 1932 issued Sept. 21, 1937 as Patent No. 2,093,423. It has been found by experience that as the dark part of the print is increased in density the transparent holes and lines become filled in by the spreading of the image, thus resulting in a major reduction in the ground noise from the dark portion of the sound track.

In accordance with the present invention, the H portion of the sound track immediately adjacent to the sound record is exposed to a light intensity sufiicient to produce the aforesaid density of 1.2 to 1.4 which is necessary for proper delineation of the high frequency components of the sound track and the portion of the opaque portion not immediately adjacent to the modulated edge is provided with an additional exposure so great that spreading of the image obscures any small clear portions which may have been due to dust or scratches on the negative and thereby eliminates substantially all of the ground noise due to the dark portion of the track.

One object of my invention is to provide a method of and apparatus for reducing the ground noise in sound film recording which is due to the unused clear portion of the track.

Another object of my invention is to provide a method of and apparatus for reducing the ground noise in sound film which is due to the relatively opaque portion of the track.

Another object of my invention is to provide a method of and apparatus for reducing the ground noise on a sound film which is due to the relatively opaque portion of the track.

Another object of my invention is to provide a method of and apparatus for reducing the ground noise on a sound film record which is due to both the unused clear portion and the unused opaque portions of the track.

Another object of my invention is to provide means for additionally exposing the opaque portion of the track to eliminate ground noise.

Other and incidental objects of my invention will be apparent to those skilled in the art from a reading of the following specification and an inspection of the accompanying drawings in which Figure l is a diagrammatic illustration of one form of sound recording apparatus in accordance with my invention,

Figure 2 shows the negative sound track made by the apparatus of Fig. 1,

Figure 3 shows a positive sound track as printed from the negative sound track shown in Fig. 2,

Figure 4 shows another form of apparatus for the performance of my invention,

Figure 5 shows a negative sound track as made by the apparatus of Fig. 4;, and.

Figure 6 shows a positive sound track as printed from the negative sound track of Fig. 5.

Referring first to the form of the invention shown in Fig. l, the recording optical system is in general the same as that shown in my aforesaid Patent No. 1,999,! 21 and includes an exciter lamp I, a condenser lens 2, an aperture plate 3 on which light from the exciter lamp is focused by the condenser lens, additional lenses 4 and 5, which focus an image of the exciter lamp I on ie galvanometer mirror 6, a second condenser lens I which collects the light emerging from the mirror 5 to the lens 5 and directed through the slit 9 in the slit plate 8 and an objective lens it which focuses an image of the slit 9 upon the film II. The lenses 2, 4, 5, 1 and Ill are, of

course, positive lenses and the lens Ii] is an achromatic lens while the lenses 2, 4, 5 and I may, if desired, be simple lenses although achromatic lenses are preferable.

The distinguishing feature of this apparatus lies in the aperture plate 3. This aperture plate is provided with an aperture 3' in the form of a triangle with a rectangular extension at its base but the two edges of the triangular end of the aperture are lined with a layer of translucent material l2. This layer I2 may consist of a shaded layer of material cemented to the surface of the aperture plate 3 as shown in the drawings or it may consist of a thin layer of partly light absorbing material cemented to the inner surface of the aperture. This latter construction is not preferred on account of the difficulty of rendering the edges exactly straight and of infinitesimal thickness. If, however, the aperture is accurately formed the construction shown in the drawings can be made to a high degree of precision. The layer I2 may, for example, absorb 50 percent of the incident light and in this case the illumination from the exciter lamp directed upon the aperture should be twice that which would be used in the usual form of apparatus. The light passing through the absorbing member I2 is therefore of the r quired intensity to produce a print density of 1.2 to 1.4 which, as described above, is the optimum value for the sound track. The light passing through the unobstructed portion of the aperture 4 is, however, twice as bright and as a consequence produces a much greater exposure in the negative in the corresponding area of the sound track; thereby causing fogging in, in the portion I8 of the print, due to the spreading of the image, of any small clear spots which might tend to be produced due to dust particles or the like upon the negative. The negative densities to produce the aforesaid. print densities are preferably 1.9 for the portion I5 and .3 for the portion 64, assuming that the customary print gamma of about" 2 is used.

The negative which is produced is shown in Fig. 2. In this figure the clear portion of the sound track is indicated at IS, the normally exposed opaque portion which is exposed through the filter I2 is indicated at I4 and the background portion exposed through the clear portion of the aperture 4 is indicated at I5. In printing this sound track on to positive film, sufficient exposure is used to print the portion I4 to the density of 1.2 to 1.4, as pointed out above, producing the portion as indicated at Iii in Fig. 3. The unexposed portion I3 thereby produces an over-exposed portion I8 correspond ing in density to the negative portion I5 which serves to obscure any clear spots in the print which might tend to be caused by dust particles, scratches on the negative, or the like. It will be apparent that the sound is produced by the light transmitted through the portion II of the positive print, the portion I6 serving as the margin thereof and transmitting approximately 5 percent of the light. This portion I6 is, of course, not sufficiently exposed so as to tend to fog in the high frequencies. The portion I8, however, as pointed out above, eliminates substantially all of the ground noise which might tend to be produced from the opaque portion of the track.

In order to prevent ground noise due to the clear portion of the positive, I may operate the galvanometer in the same manner as described in my aforesaid Patent No. 1,999,721, and particularly on page 2, lines 8 to 64. Likewise, I may operate the galvanometer 6 in the manner described hereinafter for the sound recording galvanometer in Fig. 4. Referring now to the species of the invention shown in Fig. 4, a double optical system is employed which is somewhat analogous to that shown in McDowell Patent No. 1,855,197 in Fig. 3, but which operates in an entirely different manner. The sound recording optical system which is shown at the left hand side of Fig. 4 is substantially identical with that shown in Fig. 1, including the exciter lamp I, the condenser lens 2, the aperture plate 3, the additional lens 4, the galvanometer 6, the lens I, the slit plate 8 having a slit 9 and an objective lens II] which directs the image on the film II. The only difference between this portion of the apparatus and that shown in Fig. 1 is that the translucent mask I2 is omitted from the aperture 3. However, in addition to the foregoing elements, I provide an additional exciter lamp 23, another lens 2I corresponding to the lens 2, another aperture plate 23 corresponding to the aperture plate 3 and an additional lens 2d corresponding to the lens 4 which directs the light upon a second galvanometer mirror 26 corresponding to the galvanometer mirror 6.

Electrical impulses from the microphone 30 corresponding to the sound waves impressed thereon affect the microphone amplifier 3|. Thence the impulses are first transmitted to the mixer 32 (which may be connected to as many microphones as desired for the purpose of mixing the signals) and then through the amplifier 33. Up to this point the system is a conventional sound input system. However, as in the aforesaid McDowell patent the output from the amplifier 33 is passed through the transformer 34 to the amplifier tube 35, where it is amplified and the output transmitted to a second amplifier tube 36 and the output of which is in turn passed through the rectifier 31. The output from this rectifier has the audio frequencies filtered there from by the resistors 38 and 39 and the condenser 40, and is further amplified by the D. C. amplifier GI. A variable resistor 42 is provided in the circuit to control the current output from the tube 4| which is impressed across the resistor 43. If desired, a milliammeter 44 may be inserted in the system to indicate the variations in current. Due to the action of the rectifier 3i and the filter circuit 38, 39, 49, the current from the amplifier M varies in accordance with the envelope of the sound waves and not in accordance with the sound frequencies themselves. The output from the amplifier 33, in addition to being impressed on the aforesaid circuit, is impressed on the coil 45 of the galvanometer 6 and causes this galvanometer mirror to vibrate in accordance with the sound waves thereby moving the triangular image 17 up and down across the slit 8 in accordance with the sound waves and causing a line of variable length to be impressed on the film II to produce the sound record. In addition to the winding 45, however, the galvanometer 6 is provided with a winding 48 which is connected to the output resistor 43 and this winding causes the mean position of the mirror to be shifted in accordance with the envelope of the sound waves. When there is no sound, the image ii is moved up until only the tip thereof crosses the slit 9 and only a very narrow image is produced, but when sound is applied to the apparatus the image moves downwardly in proportion to the amplitude of the sound and thereby provides room for the modulation of the line of light in accordance with the sound impulses as indicated at in Fig. 5.

The galvanometer 26 is provided with only a single Winding 49 which is connected in series with the winding 48 of the galvanometer 6 and across the output resistor 43 of the rectifying amplifier. The galvanometer 26 which produces the image 46 triangular in shape and superposed on the image 41 is, accordingly, vibrated only by the output from the rectifying amplifier and follows only the envelope of the sound waves. This superposed image 46 therefore produces the record 5| which is of normal density, while due to the superposition of the two exposures the image 50 is very opaque like the image I5 in Fig. 2. The background of the sound track 53 is, of course, left clear. In printing the negative of Fig. 5 on the positive film, the positive sound record produced has the appearance shown in Fig. 6. While the background 54 printed through the clear portion of 53 of the film is very opaque, the portion 55 which is printed through the portion 5| has the proper density of 1.2 to 1.4 which permits accurately defining the edges of high frequency sound waves, and the portion 56 printed from the portion 50 of the negative remains clear.

It will be apparent that in this form of the invention there is no possibility of high frequency impulses being interfered with by the overexposing aperture which darkens the background, as this background exposure is always kept just clear of the peaks of the sound waves while reducing the background noise from the darker portion of the track in inverse accordance with the amplitude of the sounds.

My invention is of particular importance in connection with sound recording by ultra violet light, as described and claimed in my application Serial No. 76,901, filed April 29, 1936, as the high degree of film resolution which is secured improves the definition of the images of dust, scratches and other imperfections in the same manner in which it improves the definition of the image of high-frequency sound waves.

Having now described my invention, I claim:

1. Sound recording apparatus comprising means for exposing a variable area of the sound record film in accordance with the sound vibrations to be recorded and means for additionally exposing said area with the exception of the portion adjacent the edge thereof.

2. A sound record film comprising a sound track area having an unexposed portion corresponding with the sound waves, and an exposed portion complementarily corresponding with the sound waves, the said exposed portion being additionally exposed over its entire area excepting that portion adjacent the said clear portion.

3. The method of recording sound which includes exposing a sound record to a line of light of a length varying in accordance with the sound waves to be recorded, and additionally exposing the exposed portion with the exception of a margin adjacent to the edge thereof.

4. The method of recording sound which includes exposing a sound record to a line of light of a length varying in accordance with the sound Waves to be recorded, and additionally exposing the exposed portion with the exception of the margin adjacent to the edge thereof in accordance with the envelope of the sound waves being recorded.

5. A sound recorder including a light source, an aperture having a transparent portion and a light absorbent portion, means for vibrating the beam of light passing through said aperture, means for selecting a portion of said beam, and means for directing said selected portion upon a photographic film, whereby a sound record will be produced having portions of two differing exposures, one of said exposures being sufficient to render the film substantially opaque on development and the other exposure being of less value, the margin of one of said exposed areas corresponding with the sound waves recorded.

6. A sound recorder including means for exposing a variable area of the sound record film in accordance with the sound vibrations to be recorded and means for additionally exposing said area with the exception of a margin adjacent the edge thereof, the additionally exposed portion having a contour corresponding to the envelope of the sound waves.

GLENN L. DIMMICK. 

